This invention relates to an improvement in protective devices for electrical circuits, and more particularly to an improvement in electrical fuses.
Electrical fuses are provided for interrupting electrical circuits subjected to various types of abnormal increased current flow. An abnormal increase in current flow up to six times the normal current flow is considered an overload current. Current flows of still higher magnitudes are considered short circuit currents. But for the inclusion of a fuse in a circuit, short circuit currents in the circuit may be limited very little in magnitude by the remaining circuit elements. Depending on the electrical circuit to be protected, separate electrical fuses have been provided in the past for protecting circuits against overload currents and short circuit currents. Further, in the case of overload currents, it has been found desirable to control the time delay, following the initiation of the overload current, after which the fuse interrupts the overload current flow. The limiting of such time delay may be important to protect the circuit elements, while lengthening the time delay may be desirable in circuits which are expected to experience temporary overload currents of limited duration. Such overload currents may be experienced in motor starting and upon first energizing a transformer. In electrical circuits which must be provided with protection with respect to both overload currents and short circuit currents, such protection has sometimes been provided in the past by separate fuses, one providing the protection against overload currents, and the other providing the protection against short circuit currents. Fuses have been constructed which provide both types of protection. However, the construction of some such fuses have been quite complex and therefore quite costly. Further, when such a dual purpose fuse is designed to meet particular overload and short circuit current interrupting specifications, particular components of the fuse would have to be changed to vary the overload or short circuit current interrupting characteristics.